Most disc molds can be found with flat tops. There are flat Wraiths and dome Wraiths, etc. This may have more to do with how the plastic takes form after it has been "cooked" (molded, pressed, or whatever is going on. Most of the time a flatter mold has a tighter top and will remain stabler longer. When a top is "dome" it generally is loose and has less resistance to the edge bending down with time. The faster the disc (wider edges) the more this principle applies.
Another way to check this is to observe (from side) how much height there is from the bottom edge of the disc to the side edge of the disc. Generally speaking (again), the more height the tighter the top, and the more stable the mold.